Process for the separation and liquefaction of methane and carbon dioxide with pre-separation upstream of the distillation column

Abstract

A combined plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, including a mixing means, a compressor, a first exchanger, a distillation column, a second exchanger, a separating means, an expanding means, and a separator vessel. Wherein, the mixing means is configured such that the recycle gas is the overhead vapour stream, and the first exchanger and the expanding means are combined.

Claims

1. A combined process of cryogenic separation and liquefaction of methane and carbon dioxide within a biogas stream, the process comprising: a. mixing a biogas stream with a recycle gas stream, thereby producing a mixed biogas stream, b. compressing the mixed biogas stream to a distillation pressure, thereby producing a compressed stream, c. cooling the compressed stream in a first exchanger, thereby producing a cooled stream, d. separating the cooled stream in a first separator vessel into an overhead vapour stream and a CO.sub.2-enriched liquid stream, e. cooling the overhead vapour stream, thereby producing a cooled overhead vapor stream, f. distilling the cooled overhead vapour from step d) in a distillation column thereby producing a methane stream at the top of the distillation column and a first CO.sub.2-enriched liquid stream at the bottom of the distillation column, g. liquefying the methane stream produced at the top of the distillation column in a second exchanger, thereby producing a liquefied methane stream, h. a separation step for separating the liquefied methane stream into two portions: a reflux portion and a product portion, i. expanding the first CO.sub.2-enriched liquid stream and heating the expanded first CO.sub.2-enriched liquid stream recovered at the bottom of the column in the first exchanger, and of recovering the cold from the CO.sub.2-enriched liquid stream, thereby producing a second CO.sub.2-enriched liquid stream, and j. separating the second CO.sub.2-enriched stream into a liquid CO.sub.2 stream and an overhead vapour stream, wherein the recycle gas stream corresponds to the overhead vapour stream produced in step a), further comprising, between steps f) and i), a step of introducing the CO.sub.2-enriched liquid stream from the first separator vessel into the stream of CO.sub.2-enriched liquid recovered at the bottom of the column.

2. The process according to claim 1, further comprising, upstream of step a), steps of drying and of desulfurization of the biogas stream.

3. The process according to claim 1, further comprising, upstream of step a), a step of compressing the biogas stream to the pressure of the recycle gas stream.

4. The process according to claim 1, further comprising, upstream of step a), a step of cooling the biogas stream to ambient temperature.

5. The process according to claim 1, further comprising, downstream of step j), a step of heating and vaporizing the liquid CO.sub.2 stream.

6. The process according to claim 5, wherein heating of the liquid CO.sub.2 stream is performed in the first exchanger.

7. The process according to claim 1, wherein step g) is performed by cooling the methane stream produced by means of a refrigerant fluid.

8. The process according to claim 1, wherein, in step b), the biogas stream is compressed to a pressure of between 7 and 46 bar.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

(2) FIG. 1 illustrates a refrigeration circuit in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(3) The pretreated biogas 1 (pretreated by drying, desulfurization) is introduced into the process at atmospheric pressure and temperature, it is compressed a first time in a compressor C01, to the pressure of the recycle circuit (around 8 bar). After compression, it is cooled in C01E to ambient temperature with CW (=Cooling Water) or air.

(4) Next, it is mixed with a recycle stream R, the mixture is compressed in a compressor C02, to the pressure of the distillation column (around 15 bar) or more depending on the requirements of the downstream exchanger E01 and it is cooled to ambient temperature in C02E, with CW or air.

(5) Preferably, C01E and C02E are shell and tube exchangers (cooler of the compressors).

(6) The mixture of biogasrecycle stream R is sent to the exchanger E01. This exchanger E01 firstly makes it possible to cool the mixture in preparation for the separation in the separator vessel V05 to a temperature of between ?20? C. and ?35? C., preferably between ?25? C. and ?33? C. (the objective is to remain at least 2 degrees above the equilibrium temperature of solid formation). The separator vessel V05 receives the mixture from the exchanger E01, and an overhead vapour 2 and a CO.sub.2-enriched liquid 3 at the bottom of the vessel are recovered.

(7) The overhead vapour 2 from the separator vessel V05 is then cooled to a temperature of between ?45? C. and ?55? C., preferably between ?50? C. and ?55? C., in the exchanger E01 (the objective is still to remain at least 2 degrees above the equilibrium temperature of solid formation). The cooled overhead vapour 4 can then directly supply the distillation column K01 at an intermediate stage.

(8) A heat source is used at the bottom of the column (for example an electrical resistance heater, vapour or a portion of the hot biogas in indirect contact). The product at the top of the column is pure CH.sub.4 in the vapour state. The bottom product is a liquid rich in CO.sub.2, containing around 95%-98%.

(9) The methane at the top of the column is liquefied in the exchanger E02, against a fluid from a closed refrigeration circuit. A portion 5 of the methane leaves the cycle as product and the other portion 6 (reflux portion) is used as recycle for the column and reinjected into the top of the column.

(10) The CO.sub.2-enriched liquid recovered at the bottom of the column is mixed with the CO.sub.2-enriched liquid 3 from the separator vessel V05, then is expanded and heated in the exchanger E01 countercurrent to the biogasrecycle stream R mixture.

(11) The CO.sub.2-enriched stream from the exchanger E01 is sent to the separator vessel V01.

(12) The overhead vapour of the vessel V01 is reheated in the exchanger E01 and then mixed with the biogas. It corresponds to the stream previously named recycle stream R.

(13) The liquid from the bottom of the vessel V01 is the pure CO.sub.2 7. This can, depending on the requirements, leave the process as product or be reheated in the exchanger E01 and in another exchanger E03 of the refrigeration circuit in order to be completely vaporized before leaving the cycle. Note that the pure CO.sub.2 could alternatively be reheated and vaporised in the exchanger E03 without passing through the exchanger E01.

(14) The exchanger E01 therefore uses, as sources of cold: the CO.sub.2-enriched liquid recovered at the bottom of the column and the CO.sub.2-enriched liquid from the separator vessel V05, the overhead vapour from the vessel V01 named recycle stream R at the outlet of the exchanger E01, and optionally the pure liquid CO.sub.2 recovered at the bottom of the vessel V01 in the case where the vaporisation thereof is desired.

(15) The process requires an input of refrigeration power in order to operate. This input of cold is represented in FIG. 1 by the refrigeration circuit: it is composed of: a compressor C03 with cooler C03E; an exchanger E03 which cools the compressed fluid using the recycled refrigerant fluid and the cold recovered from the separation cycle; a turbine ET01 and a JT valve PV05, for the expansion of the refrigerant fluid and production of cold; a separator vessel V02 separating the vapour and liquid phases of the refrigerant fluid; an exchanger E02 which uses the liquid phase of the refrigerant fluid to liquefy the biomethane at the top of the distillation column. The refrigerant fluid used in the scheme is CH.sub.4 but it can be replaced by other fluids such as N.sub.2, N.sub.2+H.sub.2, inter alia.

(16) This refrigeration cycle can be replaced by other sources of cold (depending on the amount of liquid biomethane to be produced). By way of example, but not exclusively: using a source of liquid nitrogen; by a Brayton cycle process.

(17) It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.